Semiconductor switches such as IGBTs or MOSFETs must be protected against overload conditions due to, for example, an overcurrent condition, otherwise excessive heating and subsequent damage to the switches can occur.
FIG. 1A is a plot of the typical output characteristics of an IGBT at 25° C., wherein the collector current (IC) is plotted against the collector-emitter-voltage (VCE). Curve 102A demarcates a first operating region 106 wherein the switch exhibits a bulk resistor voltage drop, and a second operating region 108 designated the desaturation region.
FIG. 1B is a plot of the typical output characteristics of an IGBT at 175° C., wherein the collector current (IC) is plotted against the collector-emitter-voltage (VCE). Curve 102B demarcates the first operating region 106 wherein the switch exhibits a bulk resistor voltage drop, and the second operating region 108 designated the desaturation region.
IGBTs react to an overcurrent event with an increase of the collector-emitter-voltage VCE. With increased current the VCE voltage is increased according to the bulk resistance multiplied by the collector current. If the collector current is increased further, the IGBT desaturates, and the VCE is increased until the current is limited or until VCE reaches the supply voltage (DC-link voltage).
MOSFETs react to an overcurrent event with an increase of the drain-source-voltage VDS according to the resistive voltage drop across the ON-resistance. With increased current the MOSFET saturates until the current is limited or until VDS reaches the supply voltage (DC-link voltage). For simplicity the following description uses the IGBT wording and model, but the principle is applicable for MOSFETs as well. The VCE voltage is measured in the IGBT conduction state via a high voltage blocking element. In the IGBT blocking state the high voltage blocking element decouples the sensing input from the high IGBT blocking voltage. Therefore the voltage VCE is measured in a voltage range up to approximately six to nine volts directly. Higher voltages are only detected as “high” and can lead to a desaturation event. A DESAT event is a major failure for an application, which typically requires a time consuming restart. Therefore false detection should be prevented as well as a false “no detection” of a DESAT event, which may lead to a destruction of the switch. In addition, the driver is frequently placed in a noisy environment. This leads to the need to filter the DESAT input against noise, spikes and other disturbances. This, in turn, leads to limitations on how fast the DESAT detection can occur. Due to this limitation in speed, recurring shorts for shorter duration ON pulses cannot be detected. If this condition continues for many consecutive pulses it might lead to a successive rise in temperature of the switch and eventual damage.